Members of the Nuclear Factor-κB (NF-κB)/Rel family of transcription activator proteins are tightly associated with their inhibitory proteins (I-κB) and reside in the cytoplasm. They can be induced by pro-inflammatory cytokines and are important in immunological and inflammatory processes because they direct transcription of chemoattracants, cytokines (including the NF-κB induced cytokines themselves), cytokine receptors and cell adhesion molecules. Upon induction, rel proteins dimerize and migrate to the nucleus where they activate their target genes through an NF-κB binding motif in the promoter of these genes. Examination of DNA sequences recognized by different NF-κB dimers reveals that the prefered target sites are slightly different for the existing dimer combinations of rel proteins (Chen et al., —Nature Struct. Biol. 5: 67–73, 1998; Kunsch et al., Mol. Cell Biol. 12: 4412–4421, 1992; Parry and Mackman, J. Biol. Chem. 269: 20823–20825, '94), explaining the broad variation in NFκB responsive elements that have been identified in various promoters.
Dimerization and nuclear translocation of Rel proteins is induced by a large number of agents including bacterial and viral pathogens, immune and inflammatory cytokines and a variety of agents that damage cells. An even larger number of genes appear to be targets for the activation by Rel proteins, as this family of transcription factors has been found to interact with steroid receptors such as estrogen receptors and glucocorticoid receptors, resulting in repression of target genes.
Estrogen and other steroids have profound effects on the central nervous system (1). Particularly, the ability of estrogen to modulate the brain serotonin system suggests that estrogens may play a role in the mechanism associated with depression and its treatment (2, 3). However, as ER expression is widely distributed, it is not surprising that estrogens have several other benificial effects, including the protection against atherosclerosis, Alzheimer dementia and osteoporosis. In order to limit the risk on side effects, such as an enhanced risk on breast and endometrium cancer due to treatment with estrogens, a considerable amount of effort is invested in the search for tissue-selective ER-binding compounds.
Apart from estrogens, much interest exists in tissue-selective effects of other steroid receptors. Also for these steroid receptors the development of compounds that exclusively target one set of tissues or organs (e.g. brain for psychiatric illnesses) has been hampered by the wide tissue distribution of most types of steroid receptors. For this reason much interest exists in assays that would allow for screening of steroid receptor-mediated effects in a tissue selective fashion.
The effects of estrogen are known to be mediated by two estrogen receptors (ERα and β), that belong to the superfamily of nuclear hormone receptors (15–18). The two ERs share a well-conserved modular structure. While the DNA-binding domain is highly conserved between ERα and β (96% identity) and the ligand-binding domain is relatively well conserved (58% identity), the A/B region is poorly conserved between the two receptors (20% identity). Upon ligand binding, the activated receptor dimerizes and interacts with specific DNA sequences, termed estrogen response elements (EREs), located in the regulatory region of target genes. The DNA-bound receptor can then regulate transcription either positively or negatively. It is known for ERα that the regulation of transcription is mediated by two transactivation regions: AF-1 located in the A/B domain and AF-2 located in the ligand-binding domain. The two transactivation regions may function independently or cooperate, depending on cell and promoter context (19, 20). Recently some other mechanisms have been discovered by which estrogen regulates target genes. These include genes that utilize regulatory elements as the target sequence of ER action (21) or genes that are regulated by ER through interaction with other transcription factors bound to their respective DNA-binding sites, such as AP-1 and Sp 1 (22–23, 36–38).